Adjustable aperture antenna employing dielectric and ferrimagnetic material

ABSTRACT

In order to obtain wide aperture and high directivity radiation patterns from the same antenna, the radiating structure is made of a non magnetic dielectric and a dielectric ferrimagnetic material. The magnetic part is associated with magnetizing coils energized from an aperture control current source. When the antenna is of the rod type, the rod is made of the non magnetic dielectric which is covered at least partly with a magnetic coating. When the antenna is of the spherical type, a magnetic material core is surrounded with a non magnetic dielectric shell.

Unite States Patent [1 1 Chiron et al.

[ 1 Oct. 9, 1973 1 ADJUSTABLE APERTURE ANTENNA EMPLOYING DIELECTRIC ANDFERRIMAGNETIC MATERIAL [75] Inventors: Bernard Chiron; Louis Duffau,both of Paris, France [73] Assignee: Societe Lignes Telegraphiques EtTelephoniques, Paris, France [22] Filed: July 14,1971

[2]] App]. No.: 162,444

[30] Foreign Application Priority Data 7 July 30. 1970 France 7028150Jan. 14.1971 France 7101105 [52] US. Cl 343/785, 343/787, 343/854,343/911 L [51] Int. Cl. H0lq 13/00 [58] Field of Search 343/753, 754,755, 343/785, 787, 854, 911 L [56] References Cited UNITED STATESPATENTS 3,277,489 10/1966 Blaisdell .1 343/785 3,653,054 3/1972 Wen343/787 2,869,124 1/1959 Marie 343/785 2,973,516 2/1961 Medved 343/7872,981,945 4/1961 Fyler 343/787 2,921,308 1/1960 Hansen et a1 343/787Primary Examiner-Eli Lieberman Attorney-Kemon, Palmer & Estabrook [57]ABSTRACT In order to obtain wide aperture and high directivity radiationpatterns from the same antenna, the radiating structure is made of a nonmagnetic dielectric and a dielectric ferrimagnetic material. Themagnetic part is associated with magnetizing coils energized from anaperture control current source. When the antenna is of the rod type,the rod is made of the non magnetic dielectric which is covered at leastpartly with a magnetic coating. When the antenna is of the sphericaltype, a magnetic material core is surrounded with a non magneticdielectric shell.

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saw MW 11 ADJUSTABLE APERTURE ANTENNA EMPLOYING DIELECTRIC ANDFERRIMAGNETIC MATERIAL BACKGROUND OF THE INVENTION AND PRIOR ART Thepresent invention concerns antenna structures with adjustable aperture.The requirement that the same radar equipment may be used for differenttypes of operations which become necessary in navigation and even insurveillance has led users to seek antennas whose radiation pattern canbe modified, while maintaining constant yield. An antenna structure isrequired which can simultaneously carry out a watch (very wide antennadiagram) and, when the target has been found, carry out the tracking(very selectiye radiation diagram This result can readily be obtained byusing two different antennas which are successively switched to theradar apparatus. This solution cannot be envisaged in the case ofairborne equipment or simply of mobile equipment owing to the largeoverall dimensions resulting from the use of two antennas. From theeconomic viewpoint, this solution is also unsatisfactory. It hastherefore been proposed to solve this problem with the aid of a singleantenna.

One of the proposed solutions consists in modifying the radiationpattern of the antenna by mechanically deforming the reflector element.This solution has given rise to designs which do not always provide thedesired reliability.

The present invention relates essentially to electronically controlledadjustable aperture antennas. As compared with the aforesaid mechanical'solution, they show not only increased reliability but also advantagesin regard both to weight and space requirement and to power consumptionand time constant.

BRIEF DISCLOSURE OF THE INVENTION In accordance with the essentialfeatures of the invention, the antenna is a partly dielectric, partlyferri magnetic structure associated with magnetizing coils surroundingsaid ferrimagnetic part and a control current generator feeding saidcoils. The adjustment of the radiation diagram of the antenna isachieved by changing the magnetizing current in the coils.

In a first embodiment of the invention concerning a directive structure,the dielectric part is a plain rod surrounded at least partly by aferrimagnetic coating associated with magnetizing coils. In a secondembodiment relating to an omnidirectional structure the ferrimagneticpart is an inner core surrounded by a dielectric shell arrangedaccording to Lunebergs law, said inner core being associated withmagnetizing coils. According to an additional feature of the invention,the coils associated with the ferrimagnetic part are arranged so as toprovide for latching operation, that is the operating conditions aremaintained when the magnetizing current is cut off. The latter featureis of particular interest when low power consumption is important.

DETAILED DISCLOSURE OF THE INVENTION The invention will be readilyunderstood with reference to the following description and to theaccompanying drawings, in which FIGS. 1 and 2 are diagrammaticillustrations of antennas according to the present invention.

FIG. 3 illustrates the variation of the aperture angle of a dielectricrod type antenna as a function of the length of the ferrimagneticcoating (without magnetizing current).

FIG. 4 illustrates, in the absence of magnetizing current, the variationof the gain as a function of the length of the ferrimagnetic coating.

FIG. 5 illustrates the variation of the aperture angle of the partiallycoated antenna as a function of the magnetization current.

FIG. 6 illustrates the variation of the gain of the same aerial as afunction of the magnetization current.

FIG. 7 illustrates, as a function of the current, the variation of theaperture angle of a second modified embodiment.

FIGS. 8 and 9 are two embodiments of omnidirectionnal Luneberg typeantennas according to the invention.

FIGS. 10 & 10A are a variant of the Luneberg type antenna according tothe invention which may be switched from a small aperture operation to awide aperture operation.

FIG. 11 shows the variation of the effective permeability of the corewith respect to the magnetizing field.

FIG. 12 shows the variation of the 3 dB beam width with respect tofrequency for both a Luneberg dielectric antenna and a Luneberg typeantenna according to the invention.

FIG. 13 shows the aperture angle variation with respect to the controlcurrent for the antenna of FIG. 8.

FIG. 14 shows the same variation for the antenna of FIG. 10.

FIGS. 15 and 16 show an embodiment of a latching antenna according tothe FIG. 10.

In FIG. 1, there is diagrammatically illustrated a frustoconicaldielectric structure I constituting what is usually called a dielectricrod antenna. A detailed explanation of the operation of this type ofantenna will be found in the work Les Antennes," by J. Thourel, page I88of the edition dated 1956, published by Dunod.

As is well known, the radiating structure is fed through a couplingdevice diagrammatically represented by loop 2, from an electromagneticenergy source. The base of the antenna is mounted in a metallic holder3. In'accordance with the invention, this dielectric radiating structureis at least partially coated with ferrimagnetic material and associatedwith a magnetizing device.

In FIG. 1, there is denoted by d the length of the antenna provided witha coating of ferrimagnetic material, the total length of the antennabeing L. The coating comprises two rings 4 and 5 each associated with amagnetizing winding,4' and 5 respectively. It is to be understood thatthe length d of the coating may as a variant be provided in the form ofa single piece of ferrimagnetic material surrounding the base of thedielectric rod 1. There may also be envisaged the coating of the desiredsurface of the dielectric structure 1 with ferrimagnetic material by anymethod known per se (cathode sputtering, deposition by sedimentation,etc.), the manner in which the magnetization conductors are providedbeing adapted to each case.

The diagram of FIG. 2 illustrates also a dielectric rod structure I. Itis provided with a coating of ferrimagnetic material over the whole ofits length L. As in FIG. 1, the coating consists of a set of rings 4, 5,6, 10 each associated with one magnetizing coil. The experimentaldiagrams of FIGS. 3, 4, 5 and 6 show the influence of the ferrimagneticcoating on the performance of the radiating structure. They have allbeen obtained with the same structure consisting of a dielectric rodantenna operating in the X band surrounded by a 5 mm thick ferrimagnetictore made of type 63 O7 ferrite, manufactured by L.T.T. The compositionof this ferrite corresponds to the formula 40 Fe O 9 MnO, 46 MgO, 5(TiONiO), which has been disclosed in the first addition No. 86,409 toFrench Patent No. 1,354,232, applied for on the July, 24, 1964, byL.T.T.

The diagram of FIG. 3 shows the influence on the antenna aperture angleof the length a' coated with ferrimagnetic material. In order tosimplify the diagram, there has been taken as the measure of the lengthd the rationalised value d/L, which will thus vary between 0 (noferrimagnetic material coating) and I (completely coated antenna). Inthe absence of any current magnetizing the ferrimagnetic material, aconsiderable widening of the radiation pattern is noted when the portiond of the antenna which is coated increases. The bare dielectric rodantenna has an aperture angle at 3 decibels of about 30, and whenone-fourth of the antenna is coated with electromagnetic material theaperture angle is 160. The increase in the aperture angle is extremelyrapid, and then slows down. When one-half of the length of the antennais coated, the aperture angle is 185, and it reaches 200 when theantenna is completely coated with magnetic material. 7

The curve of FIG. 4 represents under the same conditions, the variationof the antenna gain as a function of the fraction of the antenna whichis coated with ferrimagnetic material. The gain of the dielectric rodantenna is about dB, and it decreases very rapidly to 4 dB whenone-fourth of the length of the antenna is coated. The decrease slowsdown considerably and the gain changes from the value 3 dB when one-halfof the length is coated to 2 dB when the antenna is completely coated.It is current practice to characterize the yield of an antenna by theproduct G,,,,,,,. 0 .0 when the aperture angle 0, and 0 measured in twoperpendicular planes, remain small and G is the value of the gain(numerical value, while the ordinate scales of the curves are ratios ofthe gain, in dB, to the gain of a dipole). In the case of an antennawith axial symmetry such as the dielectric rod antena, 0 0 Bycalculating the values of the product G 0 from the data of the curves ofFIGS. 3 and 4, the following results are obtained It will be seen thatthe empirical law by which the yield of the antenna is related to theproduct G,.,,,,,0 shows that this yield is substantially constant up to6 50. It is well known that this formula is applicable only to smallaperture angles. More precise calculation utilising the formula whichincludes the solid angle affected by the radiation, leads to higherprecision, and when applied to the above example of embodiment it showsthat the yield of the aerial is substantially constant.

The curve of FIG. 5 shows the variations of the aperture angle ofadielectric rod antenna whose base is surrounded by a 1 cm thick tore ofthe same ferrite as stated above over percent of its height as afunction of the permeability of the said ferrite, measured by thecurrent flowing through a winding of 12 coils which is mounted directlyon the tore. As is shown, a variation of the aperture angle between and50 can be obtained. An increase of the current provides a reduction ofthe aperture angle. The curve of FIG. 6 illustrates, for the sameantenna, the variation of the gain as a function of the current suppliedto the winding. It is also possible to verify that the yield of theantenna is independent of the value of this current.

The above example concerns an antenna operating at a frequency in theneighborhood of 10 GHz. The curve of FIG. 7 has been measured on adielectric rod antenna operating in the neighbourhood of 6 GHz coatedwith a single piece of ferrite over 30 percent of its total height. Theferrite used in this embodiment is a Fe Y Gd Al garnet. The thickness ofthe coating is 1.6 cm.

The embodiment shown in the following figures is of the Luneberg lenstype. It is well known that this type of lens is a refracting structurewith a spherical symmetry and an index which varies according to thedistance to the center.

One of the main properties of these lenses is that the output from apoint source located at any point on the sphere surface is a beam ofparallel rays. As has been established, the relationship between therefractive index and the permittivity and the permeability of lensmedium is as follows Since it is quite impractical to manufacture thissphere with a continuously variable index, the practical embodiments ofLuneberg lenses are made of a central core surrounded by severalconcentric shells the constant permittivity of which decreases asincreases their distance from the center of the spherical core.

The study of electromagnetic wave propagation in a ferrimagnetic mediumis based on the property of the permeability of the medium of being atensor which means that the value of the permeability varies with thedirection considered. Theoretical considerations are very well developedin the book entitled Microwave ferrites and ferrimagnetics" by B. LAXand K. BUT- TON published by McGraw-Hill Book Co. Inc. in 1962. Asexplained page 351 the phase constant of a TEM electromagnetic wavepropagating within a non lossy medium of permeability u where has beenestablished a magnetic field H perpendicular to the propagationdirection is given by z B/B l M0 quency of the wave and H the transversemagnetic field. As seen, any modification of the value of H will producea variation of the phases ofthe wave propagating within theferrimagnetic medium. Any modification of the phase will result in amodification of the radiated lobe if the medium is used as a radiatingelement. The above is a short theoretical explanation of the reason whyit is possible to modify the aperture angle in omnidirectional sphericalantennas according to the invention.

FIG. 8 shows such an antenna made of a central core 12 of ferritesurrounded by a shell 13 made of polythene loaded with titanium dioxideThe permittivity of core 12 is c 14.9 and the permittivity of the shell3 is e 4.0. The waveguide 14 is terminated by a flange 15 the front faceof which has been shaped so as'to be adapted to the spherical shell 13.

The cut view in FIG. 8 shows the small dimension of the waveguide. Theelectric field of the microwave is within the plane of the figure. Awinding 16 surrounds core 12. It is made of four turns and is fed withthe aperture control current 1. The radius of core 12 and shell 13 arerespectively 32 mm and 41 mm. The ferrite constituting core 12 is anyttrium iron garnet (5 Fe O 3 Y O sold by L.T.T. as ferrite type 6901and by the Compagnie THOMSON C.S.F. as Type YlO.

The main characteristics of this material are as follows 411M, 1750gauss (M saturation magnetization moment) AI-l 45 to 60 oersteds tg 84.4 10* (8 loss angle) at 9 GHz.

The variation of the effective permeability of this ferrite with respectto the value of the applied magnetic field is shown in FIG. 11. Actuallyp... corresponds to the first member of equation 4.

FIG. 12 shows the variation of the aperture at 3 dB in degree withrespect to the operating frequency. Curve 51 shows the variation for aLuneberg dielectric lens of conventional manufacture and curve 52 showsthe same variation for the antenna of FIG. 1 at I 0.

FIG. 13 shows the measured curve giving the aperture of the antenna ofFIG. 8 operating at 9,375 MHZ with respect to the value of the aperturecontrol current I. At I 0, the aperture is 52, as shown on curve 52 ofFIG. 12, It is about 75 for I 2 Amperes and 100 for I 4 Amperes.

FIG. 9 illustrates another embodiment of the invention in which theantenna comprises a core 12 surrounded by two concentric shells 17 and17 made also of ferrimagnetic material surrounded by an external shell18 made of dielectric. The central core 12 and the inner shells 17 and17 have respective diameters of 32, 41 and 50 mm and are associatedrespectively with windings 19, 21 and 21 They are fed respectively withcurrents I, I and 1 which control the permeability of parts 12, 17 and17 so that the product a 6,. is equal to 12 for core 12, to 9 for shell17 and to 7.5 for shell 17 The permittivity of outer shell 18 is 2.5 andits diameter 59 mm. The values of currents I, I and I are set byexperience. The calculation of these values leads to very intricateequations because each winding establishes a field which is not limitedto the core or shell which it surrounds but extends also in theneighbouring volume of ferrimagnetic material. The variation of thevalue 6;; [LR is obtained in this embodiment by using pure YIG for thecore,.a mixture of YIG and polythene for the inner shells l7, and 17 Itis also possible to make core 12 and the two inner shells from the samemagnetic material (YIG) and to control the value of the /.L product bymeans of the currents I, 1,, 1 The embodiment shown in FIGS. 10 and 10Ais roughly the same as the embodiment of FIG. 8 except for the winding23. As shown, a central hole 22 has been drilled in the core and thewinding 23 is made of a set of semicircular turns which are each locatedin a diametral plane and which are closed through hole 22.

The core 12 is made ofa ferrite with rectangular hysteresis loop sold asferrite type 6901 by L.T.T. It has the following characteristics 41rM1650 Gauss (M, saturation magnetizing moment) AH 90 Oersted tg5 5.10 at9 GI-Iz.

The dimensions of the antenna are as follows outside diameter 41 mm,core diameter 32mm, shell thickness 4 mm, 6,; of the shell 3.75,diameter of drilled hole 5 mm.

FIG. 14 shows the variation of the aperture angle of such an antennawith respect to the control current I in winding 23 operating at 9,375MHz. As shown, the variation curve has the shape of any hysteresis loop.When a pulse of 2 Amperes is applied to winding 23 the aperture is about45 after said pulse has disappeared. On the contrary, a pulse of thesame amplitude but with the current circulating in the other directionwill set the aperture to about 100. This type of operation which isusually referred to as latching allows to switch the aperture angle fromone preset value to another value with a control pulse of a givenamplitude. The antenna does not require a continuous current to maintainmagnetization of the ferrite material as is the case in the previouslydescribed embodiments.

FIGS. 15 and 16 show the radiation diagrams corresponding to the antennashown in FIG. 10. They are measured radiation patterns.

FIGS. 15 and 16 correspond to measurements made in a plane perpendicularto the propagating direction of the microwave and containing themagnetic field of the wave as established in the wave guide 14.Measurements made in a plane perpendicular to the above plane and to thepropagation direction show that within the measurement precision, thesame values are obtained.

What we claim 1. An electrically controlled adjustable aperture r.f.antenna structure comprising:

a dielectric non-magnetic radiating unit having an axis of directivity;

a feed for said radiating unit;

magnetic dielectric means integral with said dielectric radiating unitand symmetrically arranged around said axis of directivity;

magnetizing means for said magnetic means; and

means affording connection of a DC current source to said magnetizingmeans in order to vary the aperture value of the antenna withoutchanging the directivity in accordance with the magnitude of currentsupplied to said magnetizing means.

2. An electrically controlled adjustable aperture r.f. antenna structureaccording to claim 1 in which said dielectric non-magnetic radiatingunit is a dielectric rod and said magnetic means is a sleeve coatingsaid rod.

5. An antenna according to claim 4 in which said central core consistsof an inner sphere surrounded by at least one spherical shell said coreand said shell being associated with independent coils.

6. An electrically controlled adjustable aperture r.f. antenna structureaccording to claim 1 in which said magnetic dielectric means is made ofarectangular hysteresis loop material.

1. An electrically controlled adjustable aperture r.f. antenna structurecomprising: a dielectric non-magnetic radiating unit having an axis ofdirectivity; a feed for said radiating unit; magnetic dielectric meansintegral with said dielectric radiating unit and symmetrically arrangedaround said axis of directivity; magnetizing means for said magneticmeans; and means affording connection of a D.C current source to saidmagnetizing means in order to vary the aperture value of the antennawithout changing the directivity in accordance with the magnitude ofcurrent supplied to said magnetizing means.
 2. An electricallycontrolled adjustable aperture r.f. antenna structure according to claim1 in which said dielectric non-magnetic radiating unit is a dielectricrod and said magnetic means is a sleeve coating said rod.
 3. Anelectrically controlled adjustable aperture r.f. antenna structureaccording to claim 1 in which said dielectric non-magnetic radiatingunit is a dielectric rod and said magnetic means consists of a pluralityof ferrimagnetic rings stacked on said rod near its feed end.
 4. Anelectrically controlled adjustable aperture r.f. antenna structureaccording to claim 1 in which said dielectric radiating unit is aluneberg lens and said magnetic dielectric means is the central core ofsaid lens.
 5. An antenna according to claim 4 in which said central coreconsists of an inner sphere surrounded by at least one spherical shellsaid core and said shell being associated with independent coils.
 6. Anelectrically controlled adjustable aperture r.f. antenna structureaccording to claim 1 in which said magnetic dielectric means is made ofa rectangular hysteresis loop material.